1. Field of the Invention
The invention relates to an image forming apparatus using an electrophotographic process or an electrostatic recording process, and particularly to an image forming apparatus using a developing apparatus provided with a plurality of developing devices and effecting the changeover of the developing device for effecting development on an image bearing member at the same developing position, by a rotating operation.
2. Related Background Art
Among color image forming apparatuses for forming a color image by the use of developers of a plurality-of colors, i.e., image forming apparatuses provided with a plurality of developing devices which are developing means containing therein developers including toners of different colors, there is an image forming apparatus of a construction provided with a so-called rotary developing apparatus carrying a plurality of developing devices along the outer periphery of a moving member disposed at a position opposed to a single image bearing member, wherein the moving member is rotated to thereby successively bring the developing devices close to the image bearing member.
In the image forming apparatus of such a construction, such means as latent image forming means, charging means and cleaning means acting on the single image bearing member are provided only for the single image bearing member, as compared with an image forming apparatus of an in-line type provided with image bearing members for respective colors, and this leads to a low cost and space saving.
Also, as compared with an image forming apparatus in which a plurality of developing devices are not carried on a moving member, i.e., a rotary member, but are provided along the peripheral surface of a single image-bearing member, the constructions of all the developing devices can be made the same and the developing position on the image bearing member can be set to one and therefore, the construction of the apparatus is simple and the maintainability of the apparatus can be improved.
In the color image forming apparatuses provided with such a rotary developing apparatus, there is further known a color image forming apparatus of an intermediate transfer type which can select the kinds of a recording material within a wide range, i.e., an intermediate transfer type using, as an intermediate transfer member, particularly a belt-shaped intermediate transfer member which can increase the degree of freedom with which it is disposed in the image forming apparatus, and which can achieve the downsizing and lower cost of the main body of the apparatus by the effective utilization of space.
With reference to FIG. 9 of the accompanying drawings, the general construction of an image forming apparatus adopting the electrophotographic process will hereinafter be described as an example of the above-described color image forming apparatus of the intermediate transfer type provided with the rotary developing apparatus.
In the image forming apparatus shown in FIG. 9, an ITB unit 2 having an intermediate transfer belt (ITB) 2d which is an intermediate transfer member is installed adjacent to a photosensitive drum 1 as an image bearing member, and this ITB unit 2 is constituted by the ITB 2d being stretched around a drive roller 2a, a secondary transfer inner roller 2b and a driven roller 2c. The ITB unit 2 is detachably fixed to the main body of the apparatus by a rotational position determining portion 2h and a drive roller reference portion 2g of an ITB frame 2f for supporting the rollers 2a, 2b and 2c. The ITB 2d is rotated by the drive roller 2a being rotated by a drive source (not shown) from the main body of the apparatus.
As the image forming process, the surface of the photosensitive drum 1 is uniformly charged by a charging device 1a (the charging step), and exposing means 3 (latent image forming means) exposes the charged surface to light, whereby electrostatic latent images of respective colors based on external information are successively formed (the latent image forming step), whereafter the electrostatic latent images on the photosensitive drum 1 are successively developed by developing devices 5 (5Y, 5M, 5C and 5Bk) of respective colors carried on a developing changeover mechanism (rotary) 4 which is the movable member of a rotary developing apparatus, and become developer images (toner images) (the developing step). The toner images on the photosensitive drum 1 are successively primary-transferred onto the ITB 2d in a primary transferring portion T1 by a primary transfer roller 2e which is in contact with the ITB 2d from the back side thereof, whereby a multiplex transfer image comprising the toner images of plural colors superimposed one upon another is formed on the ITB 2d. Thereupon, a recording material is fed from a sheet supplying cassette 6 to a second transferring portion T2 in which a secondary transfer inner roller 2b is disposed inside the ITB 2d and a secondary transfer outer roller 7 is disposed outside the ITB 2d opposed thereto, and the multiplex transfer image formed on the ITB 2d is secondary-transferred onto the recording material by the secondary transfer outer roller 7 (the transferring step). The recording material to which the multiplex transfer image has been transferred is sent to a fixing unit 8, where the toners thereon are fused and fixed (the fixing step), and the recording material is as an image-formed material discharged from a sheet discharging portion 9 onto a sheet discharging tray 10.
As can be understood from FIG. 4 of the accompanying drawings, which is an enlarged view of the essential portions of the present image forming apparatus, a rotary 4 which is the developing changeover mechanism of a rotary developing apparatus which is a moving member carries the developing devices 5 (5Y, 5M, 5C and 5Bk) containing therein developers (toners) of yellow (Y), magenta (M), cyan (C) and black (Bk) colors, respectively, in the named order along the outer periphery thereof in a clockwise direction as viewed in FIG. 4, and is rotated to counter-clockwise thereby move the respective developing devices 5 in succession to a position opposed to the photosensitive drum 1, i.e., a developing position 50. The toner supply to the developing devices 5 is done from supply containers 11 (11Y, 11M, 11C and 11Bk) of the respective colors adjacent to the respective developing devices 5 by the rotating operation of the rotary 4. That is, by the rotation of the rotary 4, a centrifugal force acts on the toners, and the toners are guided by a carrying projection 11d (see FIG. 11C of the accompanying drawings) provided in the interior of the supply container 11, and are carried to developing device delivery ports (not shown) provided in the developing devices 5.
Each developing device 5 is provided with a developing sleeve 51 which is a developer carrying member located on the rotational outer periphery side of the rotary 4 for carrying the toner contained in the developing device 5 to the outside. During development during which the developing device 5 is disposed at the developing position 50 opposed to the photosensitive drum 1, the developing sleeve 51 carries the toner in the developing device 5 to the electrostatic latent image formed on the surface of the photosensitive drum 1 and the developing operation is performed. At that time, a developing bias is applied to the developing sleeve 51 from a voltage source provided in the apparatus.
The developing device 5 is disposed relative to the center O of the rotary 4 in such positional relation that in terms of the angle from the center of a developing sleeve 51 to the center of another developing sleeve 51, the relative angle between the developing devices 5Y and 5M, between the developing devices 5M and 5C and between the developing devices 5Bk and 5Y is 80° and the relative angle between the developing devices 5C and 5Bk is 120°.
The reason why the angle between the developing devices 5C and 5Bk is wide is that the developing device 5Bk is higher in the frequency of use than the other developing devices 5 and therefore, the toner containing space in the developing device 5Bk or the supply container 11Bk becomes large and the carrying space therefor by the rotary 4 must also be wide as compared with the carrying spaces for the other developing devices 5.
Herein, the arrangements of the developing devices 5 in the rotary 4 are designated by the colors of the toners contained therein, and the Y portion in the rotary 4 refers to the central portion of the developing sleeve 51 of the developing device 5Y. The rotating operation of the rotary 4 is indicated by the color of the toner contained in the developing device 5 disposed at the developing position 50. For example, the description that “80° rotation for Y→M” means that the rotary member 4 has carried out the movement for the rotation by 80° so that the developing device 5 disposed at the developing position 50 may be changed over from the developing device 5Y to the developing device 5M.
The rotary 4 is usually rotated so that the developing devices 5 may be moved to the developing position 50 in the order of Y→M→C→Bk. That is, in the positional relation between the developing rotary 4 and the photosensitive drum 1 shown in FIGS. 5A to 5D of the accompanying drawings, the state is changed in the order of FIGS. 5A, 5B, 5C and 5D. In this example, as will be described later, the state shown in FIG. 5A wherein the developing device 5Y is located at the developing position 50 is defined as the home position (HP), and it is to be understood that when the power source of the image forming apparatus has risen up, the rotary 4 stands by in the state of this HP.
Also, in an image forming apparatus which, as in the above-described construction shown in FIGS. 4 and 9, is provided with a rotary, i.e., a so-called moving member 4 provided with a plurality of developing devices 5 for a single photosensitive drum 1 and in which those developing devices 5 are rotated to thereby successively form toner images on the surface of the photosensitive drum 1, as described, for example, in Japanese Patent No. 3372697 and Japanese Patent Application Laid-open No. H08-114963, as an element for effecting stable image forming, there is the setting of the distance between the photosensitive drum 1 and the developing sleeve 51 of each developing device 5 toward the rotation outer periphery of the rotary 4. Generally, even when the distance between the surface of the photosensitive drum 1 and each developing sleeve 51, i.e., the so-called SD gap is constant and the photosensitive drum 1 and the developing sleeve 51 during image forming are being rotated, the SD gap is made constant without being affected by the vibration thereof, whereby the occurrence-of uneven density can be suppressed.
Means most popular as means for relatively easily achieving the condition of making the SD gap constant is, as shown in FIG. 6 of the accompanying drawings, to provide a ramming member (runner) 52 of which the diameter is larger by a set distance in terms of the radial distance relative to the diameter of the developing sleeve coaxially with the developing sleeve 51 and of which the outer peripheral portion and the inner peripheral portion are rotatable independently of each other so that the rotation of the photosensitive drum 1 and the rotation of the developing sleeve 51 may not affect each other. Thereby, at the developing position 50, the developing device 5 is pressed toward the center of rotation of the photosensitive drum 1, and the ramming runner 52 is brought into contact with the shaft flange portion 1b of the photosensitive drum 1, whereby the set distance between the surface of the photosensitive drum 1 and the developing sleeve 51, i.e., the SD gap G, is always kept constant without being affected by the vibration due to the rotation. This means does not require to strictly adjust particularly the position of the developing device 5, and exactly the position of the developing sleeve 51, and is simple in construction itself and therefore, can be achieved at a low cost and is widely used not only in the rotary developing apparatuses.
In the image forming apparatus of the intermediate transfer type, in the image forming process, a full-color toner image is usually formed by toner images of four colors, i.e., yellow, magenta, cyan and black being successively superimposed upon one another on the ITB 2d in the primary transferring portion T1 which is a portion in which the ITB 2d and the photosensitive drum 1 are opposed to each other and in which a primary transfer roller 2e is disposed on the back side of the ITB 2d. During the time when these toner images of the first color to the fourth colors are primary-transferred onto the ITB 2d, the secondary transfer outer roller 7 is spaced apart from the intermediate transfer belt 2d. 
After the toner images of the four colors have been primary-transferred onto the ITB 2d, the secondary transfer outer roller 7 is brought into contact with the ITB 2d, and the recording material is nipped by and between this secondary transfer outer roller 7 and the secondary transfer inner roller 2b in the secondary transferring portion T2 with the ITB 2d interposed therebetween, and a secondary transferring bias is applied from the secondary transfer outer roller 7 to the back side of the recording material, whereby the toner images of the four colors on the ITB 2d are collectively secondary-transferred onto the recording material.
However, when color image forming is carried out in the image forming apparatus using the above-described rotary developing apparatus as shown in FIGS. 4 and 9, exposure and developing are effected for each color and therefore, even in a case where exposure is effected at ideal positions aligned at equal distances on the drum 1, position misregister occurs for each of Y, M, C and Bk during primary transfer, and the position misregister between the colors is recognized as color misregister.
That is, until the color toner images are primary-transferred to the intermediate transfer belt 2d from the start of the image forming, each image forming process is carried out discretely for each color. Consequently, the rotary 4, when the developing of the toner image of the first color, i.e., yellow is finished, is rotated to thereby change over the developing device 5 located at the developing position 50 from the yellow developing device 5Y to the magenta developing device 5M in order to form the next magenta toner image. In the meantime, the photosensitive drum 1 is also rotated and the yellow toner image on the surface thereof is also moved from the developing position 50, and when it has arrived at the primary transferring portion T1 opposed to the intermediate transfer belt 2d, primary transfer is effected.
When at this time, the primary transfer is effected during the rotation of the rotary 4, the ramming runner 52 of the developing device 5 comes into contact with the drum 1, and color misregister occurs due to the contact shock thereof.
Further, by the intermediate transferring process being adopted, even when the primary transfer is effected at the ideal positions aligned at equal intervals on the ITB 2d, position misregister occurs during secondary transfer and the interval is recognized as non-uniform pitch misregister.
In ordinary images, color misregister is the misregister of adjacent or overlapping colors and therefore tends to be recognized as a faulty image. On the other hand, pitch misregister, except in equal pitch images uniform in density, tends to be difficult to be recognized as a faulty image.
That is, color misregister is considered to be apt to occur if the timing at which an image of each color is formed on the photosensitive drum 1, i.e., the timing (contact timing) at which the rotary 4 changes over the developing device 5 located at the developing position 50, the timing for primary transfer and the timing for secondary transfer overlap one another.
The control of the rotation of the rotary 4 which is a developing device changeover mechanism in the image forming apparatus of the above-described construction will be described here with reference to FIG. 10 of the accompanying drawings. The rotary 4 is provided with a flag (not shown) and is designed to be rotatively moved with the flag with the rotation of a rotary motor 41. Also, an apparatus main body is provided with a home position detecting sensor 42 for detecting the rotated position of the rotary 4, and this sensor 42 is disposed so as to detect the flag provided on the rotary 4.
When a central control board 100 recognizes that the power supply switch of the apparatus has been closed, a motor table signal is sent to a motor rotation control board 43 provided in the control mechanism-of the apparatus, and the rotary 4 starts its rotating operation by the rotary motor 41. When subsequently, the signal of the home position detecting sensor 42 is detected, the motor rotation control board 43 stops the motor 41 to thereby stop the rotation of the rotary 4, and makes it stand by at the home position (HP) indicated in FIG. 5A. It is to be understood here that the HP is a state in which the yellow developing device 5Y has been disposed at the developing position 50 which is a position at which it contacts with the drum 1.
Then, when the central control board (control means) 100 of the control mechanism of the apparatus recognizes that a copy start button has been depressed, that is, an image forming signal has been transmitted, the motor 41 is rotated on the basis of two kinds of motor tables 44 (44e, 44f) shown in FIG. 13 of the accompanying drawings which will be described below.
The axis of coordinates in FIG. 13 corresponds to the number of revolutions of the motor 41, i.e., the rotational speed of the rotary 4, and the axis of abscissas indicates the rotational time of the motor 41 in which the time when the motor 41 is started is 0. In the motor tables 44 (44e, 44f) used for the control of the rotation of the rotary 4 at this time, a table 44f (solid line in FIG. 13) is used during the 120° movement of C→Bk when the color of the toner image to be formed is changed over, and a table 44e (dotted line in FIG. 13) is used during the 80° movement of Y→M, M→C and Bk→Y (HP). The acceleration/deceleration curve shown in these two tables effects the shortening of the moving time as a maximum value at which the motor 41 does not lose synchronism, and the rotational speed of the rotary 4 is equivalent and the moving time differs. That is, control is effected so that irrespective of the rotated distance, the number of revolution may be increased to the same speed, and when the rotary is rotated by 120°, the rotary may be rotated for a longer time than when it is rotated by 80°.
The timing at which the rotary 4 is rotated is set in connection with the image forming process such as exposure, primary transfer and secondary transfer, in accordance with an operation sequence shown in FIG. 14 of the accompanying drawings.
First, a laser beam is emitted to the exposing position 30 (see FIG. 4) of the drum 1 on the basis of yellow (Y) image information, and a Y latent image is formed on the drum (timing Y1 in FIG. 14). The Y latent image is moved to the developing position 50 provided downstream of the exposing position 30 with respect to the direction of rotation of the drum 1, and the Y toner is applied to the drum 1 by the Y developing device 5Y. Further, the Y toner image is moved to the primary transferring position T1 provided downstream of the developing position 50 with respect to the direction of rotation of the drum 1, and is primary-transferred onto the ITB 2d by the primary transfer roller 2e (timing Y2 in FIG. 14).
Here, the timing R1 (HP→Y), R2 (Y→M), R3 (M→C), R4 (C→Bk) and R5 (Bk→HP) of the rotation of the rotary 4 are designed to precede the exposure timing Y1, M1, C1 and Bk1 of the respective colors without fail, and when the developing step is executed, a desired latent image is formed on the surface of the photosensitive drum 1 without fail, and when the electrostatic image on the photosensitive drum 1 has come to the developing position 50, a desired developing device 5 comes to stand by earlier at the developing position 50 without fail. That position is ensured by the aforementioned contacting member 52.
However, since the transferring position T1 is located downstream of the exposing position 30 with respect to the direction of rotation of the drum 1, much time is required from after exposure has been done until the primary transfer is effected and therefore, it is impossible to change over the timing of the exposure and the timing of the primary transfer at a time and accordingly, the exposure and the primary transfer become operation sequences differing in timing from each other. That is, M image forming begins before the primary transferring step of the Y toner image ends. That is, when Y developing is terminated later than the termination of Y exposing (timing Y1), the rotary 4 starts its rotation and the Y→M rotation timing R2 of the rotary 4 comes, and the changeover from the Y developing device 5Y to the M developing device 5M at the developing position 50 is effected. In the meantime, M exposing (timing M1) is not started, but yet Y primary transfer (timing Y2) still continues and therefore, the influence of the rotation of the rotary 4 does not appear, but yet appears in the Y primary transfer.
The afore-described operation is repeated, whereby Y, M, C and Bk are multiplexly transferred onto the ITB 2d. The multiplexly transferred images on the ITB 2d are moved to the secondary transferring position T2 provided downstream of the primary transferring position T1 with respect to the direction of rotation of the ITB 2d, and are secondary transferred to a recording material conveyed in synchronism with the multiplexly transferred images, by secondary transfer rollers 2b and 7. When Bk developing is terminated later than the termination of Bk exposing (timing Bk1), the rotary 4 starts its rotation (timing R5), and the changeover from the Bk developing device 5Bk to the Y developing device 5Y is effected. In the meantime, the secondary transfer (timing X) continues and therefore, the influence of the rotation of the rotary 4 also appears in the secondary transfer.
As described above with reference to FIG. 14, the operation timing Y2, M2, C2 and Bk2 or secondary transfer timing X of the primary transfer of the respective colors overlap the rotation timing R2 (Y→M), R3 (M→C), R4 (C→Bk) and R5 (Bk→Y), respectively, of the rotary 4.
The rotary rotation timing R2 to the primary transfer timing Y2, M2, C2 and Bk2 differs in the timing affected from the timing R4 of the 120° movement of C→Bk depending on the positions of the developing devices 5 in the rotary 4 because at the timing R2, R3 and R5 of the 80° movement of Y→M, M→C and Bk→Y (HP), the rotational speed of the rotary 4 is the same as can be seen from the table 44 shown in FIG. 13 and the time required for changeover differs.
The kinds of the faulty image due to such changeover of the rotary will now be described with reference to FIGS. 15A to 15D of the accompanying drawings. For a color image, exposure and developing are effected for each color. FIGS. 15A to 15D enlarge image portions formed in the image forming processes of the respective colors, and typically show the positional relations thereof in a latent image state, a primary transfer state and a secondary transfer state. Accordingly, even when as in the latent image state shown in FIG. 15A, exposure has been effected at ideal positions aligned at equal intervals on the drum 1, Y, M, C and Bk are not formed at the same position during the primary transfer, like primary-transferred images shown in FIG. 15B, but position misregister occurs for each color, and the position misregister among the colors is recognized as color misregister.
Also, even when the color misregister as shown in FIG. 15B can be avoided and as shown in FIG. 15C, primary transfer has been effected at the ideal positions aligned at equal intervals on the ITB 2d, the respective colors are formed at the same position, like the secondary-transferred images shown in FIG. 15D, and therefore, color misregister does not occur, but yet during the secondary transfer, the position misregister in which the formed positions thereof themselves deviate from the ideal positions, and as pitch misregister which is non-uniform in the positional interval of formed images between the images, the position misregister is recognized in the enlarged images as shown.
In the ordinary images, color misregister is the misregister of adjacent or overlapping colors and therefore tends to be recognized as a faulty image. On the other hand, pitch misregister tends to be difficult to be recognized as a fault image except in equal pitch images uniform in density, unless they are such enlarged images as shown in FIGS. l5A to 15D.
The phenomena of position misregister and color misregister due to the rotational shock of the rotary which are shown in FIGS. 15A to 15D will be further described with reference to FIGS. 16A to 16C of the accompanying drawings. When as previously described, the control of the rotation of the rotary 4 is effected in accordance with the operation sequence of FIG. 14, the position misregister on the photosensitive drum 1 shown in FIG. 16A is caused by the shock with which the ramming runner 52 of the developing device 5 contacts with the drum 1 during the primary transfer. In FIGS. 16A, 16B and 16C, the number of lines indicates the image position, and 0 is the head and 74 is the trailing edge of the image. As previously described, the rotating operation time between C→Bk of the rotary 4 is long and therefore, the position misregister of the C image occurs in the vicinity of 72 lines, and along therewith, the position misregister of Y, M and Bk images occurs from the vicinity of 68 lines to the trailing edge of the image.
With C at this time as a reference color, color misregister from C is shown in FIG. 16B. Here, under the influence of the position misregister of the Y, M and Bk images, minus color misregister occurs in the vicinity of 68 lines, and under the influence of the position misregister of the C reference color, plus color misregister occurs in the vicinity of 72 lines.
A maximum color misregister amount occurring to the images on the ITB 2d is shown in FIG. 16C. Here, Bk position-misregisters toward the leading edge side of the image relative to the C reference color, and the color misregister amount thereof is minus, and M position-misregisters toward the trailing edge side of the image and the color misregister amount thereof is plus. Therefore, a greater color misregister amount occurs between Bk-M than between Bk-C and between M-C. According to this, the maximum color misregister amount occurring to the images is 0.14.
The developing device changeover operation by the rotary 4 is thus performed when the other image forming steps are being executed, whereby color misregister occurs, but to achieve an improvement in the productivity of the image forming apparatus, it is necessary to effect the changeover of the developing device during image forming, and the following inconveniences 1, 2 and 3 are apt to occur due to the shock the ramming runner 52 of the developing device 5 gives to the photosensitive drum 1.
Inconvenience 1: When during latent image forming, the ramming runner 52 contacts with the photosensitive drum 1, the uneven exposure of the latent image depicted on the surface of the photosensitive drum 1 poses a problem.
Inconvenience 2: When during primary transfer, the ramming runner 52 contacts with the photosensitive drum 1, the uneven transfer of the toner image of each color transferred to the ITB 2d poses a problem.
Inconvenience 3: When during secondary transfer, the ramming roller 52 contacts with the photosensitive drum 1, the uneven transfer of the multiplex image transferred to the recording material poses a problem.
The inconvenience 1 is avoided by setting the changeover timing of the developing device 5 by the rotary 4 so as not to coincide with the exposure timing.
Also, a flywheel is mounted on the driving shaft of the photosensitive drum 1 to thereby avoid the inconvenience 2, and the rigidity of the ITB unit 2 and the frame member 2f is enhanced to thereby avoid the inconvenience 3, but a certain degree of cost load is unavoidable.
Also, the functional unification of a color copying machine and a black-and-white copying machine has been advanced, and the amount of black toner is remarkably greater than the amounts of yellow, magenta and cyan toners, and the arrangement of the developing devices 5 in the rotary 4 is liable to become asymmetric. Thereby, the following inconvenience 4 arises.
Inconvenience 4: The positions at which the faulty images of the inconveniences 1 and 2 differ among the colors.
Due to this inconvenience 4, the position misregister among the colors is actualized and therefore the faulty image by color misregister poses a problem even if a conventional avoidance countermeasure for reducing the shock amount is taken.